Color image forming apparatus with plural color units

Abstract

A color image forming apparatus having a simple mechanism for switching plural photosensitive drum units and still maintaining a high resolution image is provided. In the apparatus, each image forming unit including each color photosensitive drum is disposed around a rotating shaft of a carriage. Each photosensitive drum has a coupling plate at one end, which is engaged by a drive side coupling plate for transmitting a drive force to the photosensitive drum, when the image forming units is moved to the image forming position. An output shaft that rotates the drive side coupling plate supports an edge of a photosensitive drum shaft in alignment. A toner image formed on the photosensitive drum is transferred onto a transfer belt, which rotates at a constant speed, one by one to form a multicolor image on the transfer belt. The multicolor image on the transfer belt is transferred onto a paper sheet. A transmission between a motor and the carriage include a worm gear.

We claim: 
     
       1. An apparatus for forming color images, comprising: a plurality of color image forming units including a photosensitive drum that has coupling member on at least one end, and a developing device;   means for moving said image forming units between an image forming position and a waiting position;   means for rotationally driving said photosensitive drum when located at the image forming position, said driving means having a coupling member that can engage removably with said coupling member of the photosensitive drum and has a rotation axis aligned with a rotation axis of the photosensitive drum;   an exposing device for exposing the photosensitive drum when located at the image forming position;   a transfer medium for accepting a toner image formed on said photosensitive drum when located at the image forming position so as to form a multicolor image including different color images transferred from said photosensitive drums; and   means for driving said transfer medium at a predetermined constant speed.   
     
     
       2. The apparatus according to claim 1, wherein registration of said photosensitive drum to said image forming position is performed by registrations of both ends of said photosensitive drum, and registration of a first end of said photosensitive drum is performed by using a member that is located at the rotation axis of said drive side coupling member for positioning said rotation axis of said photosensitive drum. 
     
     
       3. The apparatus according to claim 2, wherein registration of a second end of said photosensitive drum is performed by using a positioning member that abuts the periphery of a shaft protruded from a second end of said photosensitive drum. 
     
     
       4. The apparatus according to claim 2, wherein said drive side coupling member moves along the axis of the photosensitive drum located at the image forming position to engage or release said photosensitive drum side coupling member. 
     
     
       5. The apparatus according to claim 4, wherein the second end of the photosensitive drum is positioned prior to the first end that said drive side coupling member engages. 
     
     
       6. The apparatus according to claim 4, wherein the first end of the photosensitive drum, which said drive side coupling member engages, is released prior to the second end. 
     
     
       7. The apparatus according to claim 4, wherein an output shaft drive gear is fixed to the output shaft that rotates said drive side coupling member, said output shaft drive gear can slide axially within a predetermined distance when engaged with said drive side gear, and wherein said output shaft drive gear, said output shaft and said drive side coupling means slide axially as one unit. 
     
     
       8. The apparatus according to claim 7, wherein said output shaft drive gear is a helical gear that has helical teeth oriented in the same direction as the rotation direction of said photosensitive drum. 
     
     
       9. The apparatus according to claim 4, wherein the tip of said output shaft has a tapered convex surface, the end of the photosensitive drum has a conical concave surface, and said tapered convex surface of said output shaft is pushed axially so as to engage said conical concave surface for registration of said first side of the photosensitive drum. 
     
     
       10. The apparatus according to claim 9, wherein a thrust force that is applied by said output shaft onto said photosensitive drum located at said image forming position is received by said carriage. 
     
     
       11. The apparatus according to claim 1, wherein a tip portion of an output shaft for rotating said drive side coupling member engages the first end of said photosensitive drum so that said output shaft supports said photosensitive drum when located at said image forming position. 
     
     
       12. The apparatus according to claim 11, wherein a photosensitive drum shaft protrudes from said first end of said photosensitive drum, and said tip portion of said output shaft engages an end of said photosensitive drum shaft. 
     
     
       13. The apparatus according to claim 12, wherein said photosensitive drum shaft penetrates said photosensitive drum, and a positioning member abuts the periphery of said photosensitive drum shaft at an opposite end portion to said end that said tip of said output shaft engages. 
     
     
       14. The apparatus according to claim 1, wherein each of said plural color image forming units are removable from the machine body. 
     
     
       15. The apparatus according to claim 14, wherein each of said plural color image forming units further includes a charging device for charging said photosensitive drum, and a toner hopper that contains toner. 
     
     
       16. The apparatus according to claim 1, wherein said moving means includes a carriage that retains said plural image forming units and moves said image forming units to said image forming position one by one. 
     
     
       17. The apparatus according to claim 16, wherein said photosensitive drum is urged in a predetermined direction in said carriage, and said photosensitive drum is moved against said urging so that said photosensitive drum contacts with said transfer medium when said photosensitive drum is located at said image forming position. 
     
     
       18. The apparatus according to claim 17, wherein said carriage is pivoted on the machine body and retains said plural image forming units around a rotation axis thereof. 
     
     
       19. The apparatus according to claim 17, wherein said photosensitive drum is supported movably within a predetermined distance in said carriage. 
     
     
       20. The apparatus according to claim 19, wherein said image forming unit is supported in said carriage movably and said image forming unit including said photosensitive drum is moved in said carriage when said photosensitive drum is located correctly. 
     
     
       21. The apparatus according to claim 1, wherein said transfer medium driving means rotates said transfer medium one turn for transferring each color toner image, and the rotation ratio of said drive side coupling member to said transfer medium is an integer. 
     
     
       22. The apparatus according to claim 21, wherein the rotation ratio of a rotating member, which is connected to said drive side coupling member, to said drive side coupling member is an integer. 
     
     
       23. The apparatus according to claim 21, wherein both of said photosensitive drum driving means and said transfer medium driving means are stopped when said moving means change the image forming units at said image forming position. 
     
     
       24. The apparatus according to claim 23, wherein said photosensitive drum driving means and said transfer medium driving means are driven by a single motor. 
     
     
       25. The apparatus according to claim 1, wherein the rotation ratio of a rotation member of said transfer medium driving means to said transfer medium is an integer. 
     
     
       26. The apparatus according to claim 1, wherein said transfer medium is an intermediate transfer belt that comprises an endless loop belt, which transfers said multicolor image including different color images onto another transfer medium. 
     
     
       27. The apparatus according to claim 26, wherein the rotation ratio of a driving pulley, which drives said intermediate transfer belt, to said intermediate transfer belt is an integer. 
     
     
       28. The apparatus according to claim 27, wherein at least one guide pulley is disposed within the loop of said intermediate transfer belt, said guide pulley is located upstream of the transfer position where said intermediate transfer belt contacts with said photosensitive drum and between the transfer position and the position where said driving pulley is disposed, and the length ratio of said intermediate transfer belt to the perimeter of said guide pulley in an integer. 
     
     
       29. The apparatus according to claim 27, wherein a tension pulley is provided that applies a tension to said intermediate transfer belt, and said tension pulley is disposed downstream of the transfer position where said intermediate transfer belt contacts with said photosensitive drum. 
     
     
       30. The apparatus according to claim 26, wherein a tension pulley is provided that applies a tension to said intermediate transfer belt, and said intermediate transfer belt is urged toward the photosensitive drum at the image forming position by the tension applied by said tension pulley. 
     
     
       31. The apparatus according to claim 26, wherein said intermediate transfer belt passes around plural pulleys, a portion of said belt between pulleys contacts with the photosensitive drum at the image forming position, and said photosensitive drum rubs said intermediate transfer belt while moving for exchange of the image forming units at said image forming position. 
     
     
       32. The apparatus according to claim 31, wherein said intermediate transfer belt has an imageless area from the end position to the start position of an image, where no image is transferred, and said photosensitive drum rubs said imageless area while moving for switching the image forming unit at the image forming position. 
     
     
       33. The apparatus according to claim 31, wherein said intermediate transfer belt is stopped when the image forming unit at the image forming position is changed. 
     
     
       34. An apparatus for forming color images, comprising: a plurality of image forming units including a developing device having a different color toner and a developing roller, and a photosensitive drum;   a first motor for rotationally driving said photosensitive drum;   a carriage for retaining and moving said plural image forming units;   a second motor for driving said carriage so as to move said image forming units between an image forming position where a toner image is formed on said photosensitive drum and a waiting position one by one;   a transfer medium for accepting each color toner image formed on said photosensitive drum so as to form a multicolor image; and   said second motor also driving the developing device of the image forming unit located at said image forming position.   
     
     
       35. The apparatus according to claim 34, wherein a transmission between said second motor and said carriage includes a first clutch, a transmission between said second motor and said developing device includes a second clutch, and said second motor drives said carriage in a first rotation direction and drives said developing device in a second rotation direction. 
     
     
       36. The apparatus according to claim 35, wherein said first motor drives said photosensitive drum and said transfer medium simultaneously. 
     
     
       37. The apparatus according to claim 35, wherein said second motor drives said developing device as well as at least one of a fixing device and a paper sheet feed roller. 
     
     
       38. An apparatus for forming color images, comprising: a photosensitive drum for forming an electrostatic latent image;   a plurality of developing units including a different color toner and a developing roller;   a carriage that retains and moves said plural developing units;   means for driving said carriage so that said plural developing units are moved between an image forming position where a toner image is formed on said photosensitive drum and a waiting position one by one, said carriage drive mechanism including a motor and a transmission;   a transfer medium for accepting each color toner image formed on said photosensitive drum onto a transfer medium so as to form a multicolor image;   said transmission of said carriage driving means including a worm gear, a worm wheel and a clutch disposed between said motor and said worm gear; and   said plural developing units being retained removably in said carriage.   
     
     
       39. The apparatus according to claim 38, wherein said carriage is supported rotatably by the apparatus body. 
     
     
       40. The apparatus according to claim 38, wherein said clutch is a one-way clutch and said carriage driving means drives said carriage so that speed up and slow down control of said carriage are performed by controlling said motor. 
     
     
       41. An apparatus for forming color images, comprising: a plurality of image forming units including a developing device having a different color toner and a developing roller, and a photosensitive drum;   a carriage that retains and moves said plural image forming units;   means for driving said carriage so that said plural image forming units are moved between an image forming position where a toner image is formed on said photosensitive drum and a waiting position one by one, said carriage drive mechanism including a motor and a transmission;   a transfer medium for accepting each color toner image formed on said photosensitive drum so as to form a multicolor image;   said transmission of said carriage driving means including a worm gear, a worm wheel and a clutch disposed between said motor and said worm gear; and   said plural image forming units being retained removably in said carriage.   
     
     
       42. The apparatus according to claim 41, wherein said carriage is supported rotatably by the machine body. 
     
     
       43. The apparatus according to claim 41, wherein said clutch is a one-way clutch and said carriage driving means drives said carriage so that speed up and slow down control of said carriage are performed by controlling said motor. 
     
     
       44. The apparatus according to claim 43, further comprising; means for rotationally driving said photosensitive drum;   means for rotationally driving said developing roller;   means for rotationally driving said transfer medium;   means for feeding paper sheets; and   said motor, which supplies a drive force to said carriage via said clutch, also supplies a drive force to at least one of said photosensitive driving means, said developing roller driving means, said transfer medium driving means and said paper sheets feeding means.   
     
     
       45. The apparatus according to claim 44, wherein said drive force is transmitted from said motor to at least one of said photosensitive driving means, said developing roller driving means, said transfer medium driving means and said paper sheets feeding means via a second one-way clutch, and one of said first and second one-way clutches is engaged depending on the rotation direction of said motor. 
     
     
       46. The apparatus according to claim 41, further comprising; means for detecting carriage position, each carriage position corresponding to each image forming unit retained in said carriage, said detecting means generating plural kinds of position signal; and   means for controlling a stop position of said carriage by controlling said motor to stop at a predetermined rotation angle after said position signal is generated by said detecting means.   
     
     
       47. The apparatus according to claim 46, wherein at least one of said carriage positions detected by said detecting means can be distinguished from other carriage positions using said position signal. 
     
     
       48. The apparatus according to claim 46, wherein controlling of said motor includes a speed up period and a slow down period, and said detecting means are disposed so as to detect said carriage positions during said slowdown period. 
     
     
       49. The apparatus according to claim 46, wherein said detecting means also indicates whether the corresponding image forming unit is installed or not.